Thursday, January 8, 2009

This is the first in a series of articles about my experience with the MOST (Microvariability and Oscillations of STars) team studying Betelgeuse in which BatmanMang does not summon Michael Keaton ... but rather we take a closer look at one of the brightest and most recognized stars in the night sky which is pronounced very nearly as beetle juice.

Below are some interesting images of Betelgeuse, an introduction to MOST's unusual imaging method, and an update on some facts about this remarkable star.

Betelgeuse (α Orionis) is the 9th brightest star in our night sky visible in both the northern and southern hemispheres. Despite its alpha (α) designation, Betelgeuse is the 2nd brightest star in Orion after Rigel (it's variable and almost ties Rigel at its brightest). It's a red super giant much younger, heavier, and larger than our own Sun. It pumps out an enormous amount of energy and is expected to end its life in a spectacular supernova. Living fast and dying young.

Betelgeuse Photo Gallery

Our first image of Betelgeuse is a stunning photo of Orion and Mars and Monument Valley taken by Wally Pacholka (a former Scout and Scouter). Betelgeuse is the red star dead center. The familiar belt is vertical and the glow of the Orion nebula is clearly visible in the sword. Mars dominates the upper left.

Next is the Hubble Space Telescope image from APOD taken in 1995 and the first image of the face of a star other than our Sun. The image is a false colour image combining several images taken with two different filters. The extreme difficulty of imaging the face of a star as more than a point can be seen in the pixel structure visible in the image. A large hot spot is visible on the disk.

Whoa! What's this? It's not actually a direct image but a
false colour 3D representation. MOST sees only intensity. The
terrain and color effects are just ways to emphasise the strength of the
signal. The flat image to the right of the 3D version is closer to what
MOST actually sees. These images are "stacks" of 30 individual
exposures combined to form a single image. But what's actually behind
the image?

Focus, Focus, Focus

Stars aren't donuts so why do these images look like one? Anyone who has ever focused any kind of reflecting telescope may be able to guess. The shape is a result of the telescope's construction (shown below). The telescope, a Rumak-Maksutov design, uses a compound optical path to
get both a longer focal length and higher magnification. The primary mirror is donut shaped so the light can be reflected back through the hole to the imaging systems. This by itself won't cause the donut image. In perfect focus, a star will still image as point. But MOST isn't perfectly focused and that's why we see the donut.

Credit: MOST Team, CSA and UBC.

Wait, didn't the US Hubble Space Telescope have a focusing problem requiring a Shuttle mission to service the scope and install "glasses" to correct the problem? Surely we Canadians weren't doing a "me too"?! Why would anyone want to their images to be out of focus?

Firstly, MOST isn't as much a telescope as a photometer so it was never intended to take pretty pictures. Secondly, defocusing a star spreads its light out from a point inside one pixel over several (about 25) pixels. This has two important advantages:

The intensity of light reaching individual pixels is reduced and they are less likely to be blinded by bright stars which would otherwise require much higher "shutter" speeds than the CCD is capable of.

By using measurements from all of the pixels exposed for each guide star, MOST can effectively calculate the position of those stars to within a fraction of a pixel! Without this MOST could not maintain its rock steady aim.

The Power and Sensitivity of MOST

Weirder still is the array of small Fabry microlenses that spread the light even further. To get the best data, MOST must keep the target star dead center in one of its Fabry lenses. The microlens produces an image of the mirror of the telescope, spread out over an area of about 1500 pixels (π * (25)^2 minus the area of the hole in the mirror).

The mirror is illuminated by the light of Betelgeuse so that starlight is spread evenly over the same area on the CCD. In another part of the focal plane, not under a Fabry microlens, each pixel would represent 3 arcseconds across the sky. But under the Fabry array, the sky is no longer being imaged. The telescope entrance pupil (the corrector in a Maksutov design) is projected in focus, and it is through that pupil that the star light is passing.

So in a sense, MOST is taking pictures of itself, lit up by Betelgeuse.

Not only does the defocusing prevent MOST from being blinded by the brightest stars, it gives MOST enormous sensitivity to detect very subtle variations in the stars it's tasked with watching. I attended a talk by Dr. Jaymie Matthews the Principal Scientist for MOST at the Mississauga RASC's December meeting. He has some wonderful analogies one of which puts the 1 ppm (part per million) sensitivity of this instrument into perspective. Imagine an image of the Empire State Building at night. MOST can detect a change in intensity of the light equivalent to raising or lowering one blind by 3 cm!

David (Mang) and Jaymie at the RASC presentation.

Some Awsome Betelgeuse facts

There is always a lot going on with Betelgeuse and we should expect it to continue to be full of surprises.

Imaging Betelgeuse

Betelgeuse was the first star, other than our Sun, to have its disk imaged!

The first non-optical "images" of its disk were obtained in 1975 using the 4.0m Mayall telescope at Kitt Peak.
This was later followed with the first true optical image taken by the
Hubble Space telescope. These showed that Betelgeuse was far from
uniform with a mottled surface, massive spots varying in temperature
and brightness.

Other images of Betelgeuse have been made in infra-red, radio, and ultraviolet light (seen below):

Credit: Andrea Dupree, Ronald Gilliland,CfA, STScI, NASA, ESA

Update: The next image was produced using the ESO's 8m Very Large Telescope with so-called lucky imaging.

A 2008 study using the Very Large Array
of Radio Telescopes determined that Betelgeuse is further from us than
previously thought. This also means that Betelgeuse is proportionally
larger than previously thought!

Prior to this Burnham's Celestial Handbook, a standard reference text, gave the distance to Betelgeuse as about 520 light years with a diameter ranging from 550 to 920 solar diameters and a mass of about 20 solar masses. Older sources gave it as 427 light years.

The new distance is calculated as 640 light
years (although allowance for error gives a range of 595 to 790 light
years). Any variation due to error means that Betelgeuse is larger (or smaller) by the same proportion.

The method used to find the distance to Betelgeuse is
called the parallax method and involves measuring the apparent movement
of a star from Earth at six month intervals using Earths orbit around
the Sun to establish the longest possible base line for triangulation.
Astronomers measure such distances in units called Parsecs which are then converted to light years (at 3.26 Light Years / Parsec). This only works for stars that are relatively close to us and depends upon our distance to the Sun.

Astronomers often measure the size of stars in angular diameters as this avoids confusion when a distance is revised. Ignoring the fact that Betelgeuse varies in size and using a typical size (angular diameter) of 0.045" we can see that with this new distance the star is very much larger at 925 versus 750 times the size of our Sun.

To put this in perspective, if Betelgeuse replaced our sun, it would swallow everything out to past Jupiter. Because there is a range and because Betelgeuse is variable and swells, Saturn too might be doomed.

Variable Brightness and Size

Betelgeuse is a variable star that dims and brightens over a period of 5.7 years ranging from magnitude 0.2 to 1.2. There is also a lesser cycle of between 150 and 300 days. Observations of Betelgeuse have shown that this dimming and brightening goes hand in hand with physical shrinking and swelling of the star. Betelgeuse can expand by almost 60% in size!

Stranger still, Betelgeuse appears to be different sizes when looked at with different frequencies of the electromagnetic spectrum! In fact, under some wave lengths it's almost twice as large in diameter as in visible light. The image below from 1998 is just beyond the far infra-red and shows the size of the disk in visible light as well as comparing the size to the orbits of Jupiter and Saturn.

Credit: National Radio Astronomy Observatory (NRAO)

Note: the lines showing the orbits of Jupiter and Saturn are based on the older smaller distance - they are too big! Saturn's orbit should run from +/-50 milliarcseconds.

Betelgeuse's enormous volume vastly overwhelms its high mass and results in a photosphere that is so thin that it has been described as a red hot vacuum. The outer layers of the star may be even more interesting consisting of an extended chromosphere and dust clouds that grow and shrink with the star.

The image below clearly shows four companions labeled B through E (Betelgeuse itself is A). Most of these stars have been known since the time of Herschel.

Credit: Peter Wienerroither, reprinted with permission.

To give some idea of scale, the distance from Betelgeuse to the E star at 175 arcseconds should be about 0.5 light years based on the latest distance estimates.

It's easy to see that these could be overlooked. Unlike well known binary systems, such as Albireo, these companions are quite dim ranging from 11 to almost 15th
magnitude. For comparison, if a duplicate of our Sun orbited
Betelgeuse it would appear about as bright as the brightest of these
stars. And although several star catalogs list Betelgeuse as a multiple
star system, Burnham states that it isn't certain if these are just
field stars (or if they form a true multiple star system).

The image above also shows labels for an "a" and "b" star; although they are not visible being hidden in the glare of the main star. These two companions were reported in 1985 based on observations using speckle interferometry. According to the study the "a" star orbits Betelgeuse at about 60 milliarcseconds and "b" orbits at about 500 milliarcseconds. This places the "a" star inside the Betelgeuse's complex outer layers. To my knowledge these observations were never reproduced and many astronomers are doubtful that this "a" star exists.

Spots!

Beteleguese is believed to have massive sunspots that have been apparent in images of the disk. Even the first image taken by the Hubble in 1996 showed evidence of a bright region. Recent images obtained from Infrared Optical Telescope Array (IOTA) interferometer on Mount Hopkins in Arizona and the Paris Observatory (LESIA) show two massive spots. The spots are larger than the distance from the Earth to our Sun!